This invention relates to a computer assisted surgery system for use in inserting interlocking screws in an intramedullary rod.
A current surgical treatment for fractures of the shaft of long bones (e.g., femur and tibia) is the insertion of an intramedullary rod (IM rod). These devices are relatively rigid devices inserted into one end of the bone and down the center canal of the bone shaft, such that the fracture site is bridged. Transverse holes in either end of the IM rod receive screws inserted transversely through the bone in order to lock the two bone fragments relative to one another. The insertion of the screws farthest from the IM rod insertion hole is currently a difficult and time consuming procedure requiring numerous x-ray images. An intraoperative x-ray machine (C-arm) is repeatedly fired and reoriented until it is exactly aligned with the transverse holes as evidenced by x-ray images displaying the holes as “perfect circles”. To establish a starting point, the surgeon uses further x-ray images to align the drill tip with the images of the holes. The surgeon then uses the source-to-receiver axis of the C-arm as an external reference frame along which the long axis of the drill is oriented. Even after this, several attempts may be required to drill the holes into the bone and through the transverse holes.
Several alternative approaches have been employed in an attempt to speed this process. External jigs have been tried with little success because inaccuracies in the jig, inaccuracy of the mounting between jig and IM rod, and deformation of the IM rod accumulate to cause the final jig hole positions to be unreliably aligned with the IM rod holes. Radiolucent drills and drill guides and laser sighting devices have been developed which, in the best cases, improve the speed and accuracy of hole placement, but still require a significant number of x-ray images to be obtained in order to first achieve a C-arm orientation that produces “perfect circles” in the images.
Image-guided approaches have been developed, but these too require the “perfect circle” alignment of the C-arm. Most image guided systems display the drill trajectory over “perfect circle” images of the IM rod. One system assists the surgeon in correctly orienting the C-arm to obtain “perfect circles”: “Surgical Navigation Based on Fluoroscopy Clinical Application for Computer-Assisted Distal Locking of Intramedullary Implants”, Suhm, et.al., Computer Aided Surgery 5:391-400, 2000. Another difficulty with existing image guided systems is that the surgeon must align the drill guide while viewing an “end on” representation of the drill guide, which can be quite challenging.
Several devices have been described (U.S. Pat. Nos. 5,411,503, 5,540,691, 6,074,394, 6,081,741) in which an emitter is inserted into the IM rod, down to the level of the interlocking holes, and transducers on the drill guide report the position of the drill trajectory relative to the holes. These devices, however, require equipment dedicated to this one surgical task, require the extra step of inserting an emitter to the level of the hole, and typically provide only rudimentary “end on” representations of the drill trajectory.
U.S. Pat. No. 6,285,902, incorporated herein by reference, entitled “Computer Assisted Targeting Device for Use in Orthopaedic Surgery” describes a system in which, preferably, orthopaedic surgical tools outfitted with infrared LEDs are tracked by an optical localizing device. The poses of these tools are determined and graphic representations of the tools are superimposed on standard intraoperative x-ray images. This allows the surgeon to view, in real time, the position of the tool or tools with respect to an imaged body part or another tool or tools. In the preferred embodiment, a drill guide outfitted with infrared LEDs is tracked and the trajectory of its bore is displayed on the x-ray image of the involved bone. This allows a surgeon to accurately predict the trajectory of a guide pin that passes through the bore of the drill guide. The guide pin, once inserted, is used as a reference for the insertion of implantable cannulated screws.
An alternative embodiment of the previous invention, described in the referenced patent, allows its use in the insertion of distal interlocking screws in an intramedullary (IM) rod by displaying the drill guide trajectory relative to a computer generated representation of a cross-section of the IM rod. The current invention is an enhancement to the previous invention that adjusts the graphic representations of the IM rod based on information developed from the x-ray images. This facilitates the more accurate alignment of a drill through the holes and eliminates the need to align the x-ray beam with the holes in the IM rod. This can significantly reduce the amount of radiation involved in the procedure and reduce the time required to insert the screws.